*************************** ** THE GEM MESSENGER ** *************************** Volume 3, Number 13 September 23, 1993 ------------------------------------------ GEM Mini-Workshops at the Fall AGU Meeting ------------------------------------------ This year's GEM mini-workshops will be held on December 5, the Sunday prior to the Fall AGU meeting in San Francisco. Two rooms will be reserved from 1 to 6pm for working groups 1, 2, 3, and 5. Please plan to attend and make your travel reservation accordingly. More details about the workshops will be sent out in the next GEM Messenger. -------------- GEOTAIL REBORN -------------- By A. Nishida (ISAS) While the Geotail satellite has been yielding beautiful data since its launch on July 24, 1992, one important item has been missing. The Low Energy Plasma Analyzer ceased to function on August 22 after demonstrating its high capability during two days of the normal performance. The cause of the malfunction was the latch up of ICs that are at the interface between the onboard data handling unit and this instrument. Presumably there was an electric arching. It was clear that the latch up could be released if the power supply to these ICs was turned off, but the spacecraft was so designed that this could only be done by turning off the power of the entire spacecraft. The Geotail team immediately formed a task force to investigate possible means of the power shutdown and its potential effect on the spacecraft. After an extensive study the task force came to the conclusion that the power shutdown could best be realized in the lunar eclipse and that there was little likelihood that any instrument would be damaged by the operation. The proposal to perform the shutdown operation was made to the Joint Working Group meeting held at ISAS on June 10, 1993, and it was accepted by a ballot of the project scientists and the principal investigators. The delta-V maneuver to correct the orbit was performed on June 18, 1993. Originally the Geotail orbit had no shadow periods during the distant tail phase that is scheduled to last until late 1994. The maneuver was to put the spacecraft in the lunar eclipse around 1430 to 1500 UT (2330 to 2400 JST) on September 1, 1993. The important constraint in designing the eclipse was the communicability of the spacecraft with the 64 m antenna of ISAS at Usuda in central Japan. On the day of the eclipse, the power was turned off by separating the battery from the power system at 2325 JST. The power returned to the satellite while the spacecraft was still in the penumbra, and the communication with the ground was resumed at 2338 JST. Since then, it took two weeks to restart the operation of the entire science instruments, as the onboard program had to be reloaded. This phase was over by September 14, 1993. All the science instruments have been turned on, and we have not noticed any anomaly in the quality of the data. The Low Energy Plasma Analyzer has come back. We still have difficulty with the operation of the ion mass spectrometer, but the ion and electron energy analyzers and the solar wind detector have started to send beautiful data again after having had a good rest for slightly more than a year. The only casualty of the operation was a slight (42 micro- seconds) phase delay that was produced between the signals of the two onboard editor systems. This does not affect the acquisition of the science data in any way, but it has made it necessary to perform the temperature control of the hydrazine pipelines in the attitude-orbit control unit with greater care than before. This is because the temperature data is affected by this asynchronization when the spacecraft is operated in the routine mode where both editors are used in parallel at different data rates. We have been testing a new scheme to keep this temperature within limits and all indications are that the new scheme works. I thank all who lent their force to the success of this operation and deeply acknowledge the understanding of our colleagues. Interest and support expressed by many individuals in the international STP community have been most encouraging. September 15, 1993 +-------------------------------------------------------------------------+ |To add name to the mailing list, send a message to : guan at igpp.ucla.edu | |For message to whole GEM mailing list, send to: gem at igpp.ucla.edu | |For message to a specific working group, send to: | | gem_field at igpp.ucla.edu (WG1); gem_boundary at igpp.ucla.edu (WG2); | | gem_current at igpp.ucla.edu (WG3); gem_data at igpp.ucla.edu (WG4) | | gem_ggcm at igpp.ucla.edu (WG5) | | gem_chair at igpp.ucla.edu (WG chairs, T. Eastman and W. Lotko) | |GEM Bulletin Board: telnet terra.igpp.ucla.edu; user name: gem or GEM | | contents: GEM Messengers, magnetic indices, IMF data | |Next Meetings: WG5 Meeting, Phillips Lab., October 26,27,1993 | | GEM Workshops, San Francisco, CA, December 5, 1993 | |Please update your e-mail address. | |CAUTION: Do not send messages to gem at igpp.ucla.edu unless you want | | your message to go to everyone in the GEM mailing list! | +-------------------------------------------------------------------------+